Debugging system and debugging method for medium and long term heat removal system under severe accident condition of nuclear power plant

Abstract

The present application relates to the field of nuclear power, in particular to a debugging system and a debugging method for a medium and long term heat removal system under severe accident conditions of a nuclear power plant. The debugging system is characterized in that: the cooling water pipeline of the cooling water side medium and long term heat removal system is connected in parallel with the cooling water side pipeline of the original equipment cooling system; the steam pipeline of the secondary side passive cooling system of the steam generator is connected with the main steam pipeline of the steam generator, and the condensate water pipeline is connected with the feedwater pipeline of the steam generator; the heat exchanger of the passive cooling system is connected with the steam pipeline and the condensate water pipeline respectively. The debugging method comprises debugging of the cooling water side medium and long term heat removal system and debugging of the secondary side passive cooling system of the steam generator. The present application greatly reduces the debugging cost and the time required for debugging of the medium and long term heat removal system, and the method takes into account the requirements of environmental protection, energy saving and emission reduction.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of nuclear power, in particular to a debugging system and debugging method for a medium and long term heat removal system under severe accident conditions of a nuclear power plant. BACKGROUND

[0002] Under severe accident conditions, the heat removal capacity of a nuclear power plant is very important. In most power plants, emergency water supply facilities, emergency diesel engines and mobile power sources are added to increase the ability of the power plant to cope with severe accidents. For a newly built nuclear power plant after an accident, some new systems are added during the design phase. These systems can guide the heat of the spent fuel pool under severe accident conditions through active or passive means, greatly increasing the ability of the power plant to cope with severe accidents.

[0003] The medium and long term heat removal system used in a nuclear power plant mainly adds a new system to guide the heat of the equipment and spent fuel pool to the air under severe accident conditions such as tsunami and earthquake, which results in the loss of seawater for equipment cooling. Since only the heat of the spent fuel pool and related systems and equipment needs to be guided under severe accident conditions, the amount of heat is much lower than that required during normal operation, which makes it possible to guide the residual heat of important equipment such as the spent fuel pool through mechanical ventilation cooling towers under extreme conditions.

[0004] To save the cost of the newly added medium and long term heat removal system of a nuclear power plant, these systems usually use the existing equipment cooling water system pipes to directly connect the newly added medium and long term heat removal system to some important users of the equipment cooling water system, replacing the equipment cooling water system to cool the spent fuel pool and other important heat sources under extreme accident conditions. The other side is cooled by a mechanical ventilation cooling tower, which replaces the important service water system under extreme accident conditions (such as loss of seawater) and reduces the dependence on seawater and other water sources under extreme accident conditions, meeting the heat removal requirements of a nuclear power plant under severe accident conditions.

[0005] The addition of these new medium and long term heat removal systems under severe accident conditions forces the nuclear power plant to increase the debugging plan and debugging work for the medium and long term heat removal system during the installation and debugging phase. In order to verify the heat removal capacity of these new systems, sufficient heat sources are usually required. However, during the installation and debugging phase of the unit, there are few high-power heat generating devices or conditions, so external heat sources such as external boilers need to be purchased for the debugging work of these new systems. These external heat sources require a large amount of additional costs, greatly increasing the debugging cost of the new power plant. SUMMARY

[0006] The technical problem solved by the present application is to provide a debugging system and a debugging method for a medium and long term heat removal system under a severe accident condition of a nuclear power plant, which greatly reduces the debugging cost and the time required for debugging of the medium and long term heat removal system, and meanwhile, the method takes into account the requirements of environmental protection and energy saving and emission reduction.

[0007] The present application provides a debugging system for a medium and long term heat removal system under a severe accident condition of a nuclear power plant, wherein a cooling water pipeline of the cooling water side medium and long term heat removal system is connected in parallel with a cooling water side pipeline of an original equipment cooling system;

[0008] A steam pipeline of a passive cooling system on a secondary side of a steam generator is connected with a main steam pipeline of the steam generator, and a condensate water pipeline is connected with a feedwater pipeline of the steam generator;

[0009] The passive cooling system heat exchanger is connected with the steam pipeline and the condensate water pipeline respectively.

[0010] In a specific embodiment of the present application, the residual heat removal system is connected with a primary loop, and comprises a residual heat removal pump and a residual heat removal system heat exchanger,

[0011] In the cooling water side medium and long term heat removal system, the cooling water pipeline is connected with one side of a medium and long term heat removal system cooling water heat exchanger, and forms a first circulation loop with the residual heat removal system heat exchanger,

[0012] The other side of the medium and long term heat removal system cooling water heat exchanger is connected with a ventilation cooling tower fan through a pipeline to form a second circulation loop.

[0013] In a specific embodiment of the present application, the original equipment cooling system comprises a heat exchanger and a connecting pipeline, and forms a third circulation loop with the residual heat removal system heat exchanger,

[0014] The cold water output by the heat exchanger is subjected to heat exchange in the residual heat removal system heat exchanger, the hot water obtained by the heat exchange is returned to the heat exchanger, and the heat exchange is subjected to heat exchange again in the heat exchanger, and the heat exchanged cold water is again introduced into the residual heat removal system heat exchanger.

[0015] In a specific embodiment of the present application, second and third valves are arranged on the pipelines on both sides of the medium and long term heat removal system cooling water heat exchanger on the first circulation loop.

[0016] First and fourth valves are arranged on the pipelines on both sides of the residual heat removal system heat exchanger on the third circulation loop.

[0017] In a specific embodiment of the present application, the steam pipeline is further connected with a nitrogen pipeline, a fifth valve is arranged on the steam pipeline, a sixth valve is arranged on the condensate water pipeline, and a seventh valve is arranged on the nitrogen pipeline.

[0018] The application provides a debugging method for a medium and long term heat removal system under a severe accident condition of a nuclear power plant, comprising: debugging of a cooling water side medium and long term heat removal system and debugging of a passive cooling system on a secondary side of a steam generator;

[0019] The debugging of the cooling water side medium and long term heat removal system comprises the following steps:

[0020] Step 1-1: After a late stage of a thermal test of a nuclear power plant unit, gradually reduce a primary loop temperature to a temperature at which a residual heat removal system operates or to an upper limit value as close as possible to a spent pool cooling temperature under a severe accident condition through the steam generator when the unit state is descending, so as to simulate a real condition;

[0021] Step 1-2: When the primary loop temperature is reduced to the temperature at which the residual heat removal system is put into operation, put the residual heat removal system into operation and start a residual heat removal pump;

[0022] Step 1-3: When the primary loop temperature is close to a designed highest temperature of the spent pool under the severe accident condition, close a third circulating loop pipeline, open a cooling water pipeline of the cooling water side medium and long term heat removal system, and realize heat exchange through the cooling water pipeline of the cooling water side medium and long term heat removal system and a residual heat removal system heat exchanger;

[0023] Step 1-4: Check a primary loop temperature change, check inlet and outlet temperatures of the residual heat removal system heat exchanger, and check a cooling water side medium and long term heat removal system temperature and equipment operation;

[0024] The debugging of the passive cooling system on the secondary side of the steam generator comprises the following steps:

[0025] Step 2-1: Use main steam to provide heat for the passive cooling system on the secondary side of the steam generator;

[0026] Step 2-2: Monitor a water level change of the steam generator and a primary loop temperature;

[0027] Step 2-3: Determine a heat exchange efficiency of a passive cooling system heat exchanger through calculation of a main pump heat release, a steam pipeline flow, a condensate flow and a temperature.

[0028] In a specific embodiment of the application, the step 1-1 further comprises the following steps before the step 1-1:

[0029] Water is filled in the first circulating loop and the second circulating loop and air is exhausted;

[0030] Circulating cooling water is established for the first circulating loop and the second circulating loop;

[0031] A ventilation cooling tower fan is started;

[0032] It is confirmed that all equipment of the cooling water side medium and long term heat removal system is started without any abnormality and the circulating cooling water has been established.

[0033] In one embodiment of the application, the step 1-2 further comprises:

[0034] Close the atmospheric release valve, and transfer as much heat generated by the main pump as possible to the long-term heat removal system through the waste heat exchanger.

[0035] In one embodiment of the application, before the debugging of the steam generator secondary side passive cooling system,

[0036] Nitrogen is filled in the steam pipeline of the passive cooling system exchanger;

[0037] Demineralized water is filled in the condensate pipeline of the passive cooling system exchanger.

[0038] In one embodiment of the application, the step 2-1 specifically comprises:

[0039] During the thermal test of the unit, close the atmospheric release valve when the primary loop temperature approaches the real operating temperature;

[0040] Use the main steam to discharge nitrogen in the steam pipeline;

[0041] Use the main steam to provide heat for the steam generator secondary side passive cooling system.

[0042] Compared with the prior art, the debugging system and method of the long-term heat removal system of the nuclear power plant under severe accident conditions has the following beneficial effects:

[0043] (1) A debugging method for the newly added long-term heat removal system of the nuclear power plant is proposed, which combines the debugging work of the long-term heat removal system with the original debugging work of the power plant, without the need to create additional test conditions for the long-term heat removal system, greatly saving the debugging time of the newly added long-term heat removal system of the nuclear power plant. Through this method, the test can be completed in 10-12 hours during the thermal test of the nuclear power plant. If an external heat source is used, it will take 5-7 days to complete the test due to the more preparation work. Therefore, this method can save 4-6 days for the unit debugging period.

[0044] (2) No additional heat source needs to be purchased and installed for the debugging work of the long-term heat removal system, reducing the equipment cost and labor cost of the long-term heat removal system debugging. Taking a million-kilowatt pressurized water reactor as an example, 500,000 yuan of equipment cost can be saved, and 2 system engineers can save 4-6 days of work.

[0045] (3) Make full use of the pipeline between the new system and the original system of the power plant, and introduce the heat generated by the main pump during the thermal function test of the nuclear power plant into the new system, to truly test the heat exchange capacity of the new system, so that the test result is more accurate;

[0046] (4) Ingeniously use the heat generated during the thermal function test of the nuclear power plant, and do not use additional heat sources such as boilers to debug the medium and long-term heat removal system, which has the advantages of energy saving and environmental protection. At the same time, the method of the present patent is helpful to promote the modification of the commercial power plant on the basis of the original, increase the medium and long-term heat removal system to improve the ability of the power plant to deal with severe accident conditions, and help the development of pressurized water reactor nuclear power plant in the direction of safety and environmental protection. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The structure diagram of the debugging system of the medium and long-term heat removal system under the severe accident condition of the nuclear power plant is shown in the figure.

[0048] In the figure, 1 is a reactor, 2 is a steam generator, 3 is a main pump, 4 is a residual heat removal pump, 5 is a residual heat removal system heat exchanger, 6 is an original equipment cooling water heat exchanger, 7 is a cooling water pipeline inlet, 8 is a cooling water pipeline outlet, 9 is a medium and long-term heat removal system cooling water heat exchanger, 10 is a circulating pump, 11 is a mechanical ventilation cooling water circulating pump, 12 is a ventilation cooling tower fan, 13 is a first valve, 14 is a second valve, 15 is a third valve, 16 is a fourth valve, 17 is a fifth valve, 18 is a sixth valve, 19 is a passive cooling system heat exchanger, and 20 is a seventh valve. DETAILED DESCRIPTION

[0049] In order to further understand the present application, the embodiments of the present application are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present application, but not limiting the present application.

[0050] In the present application, the medium and long-term heat removal system includes a cooling water side medium and long-term heat removal system and a steam generator secondary side passive cooling system.

[0051] The method makes full use of the characteristics that the medium and long-term heat removal system of the nuclear power plant is connected with the original equipment cooling water pipeline of the power plant in system arrangement. During the thermal test stage of the nuclear power plant, the main pump is usually kept running continuously to simulate the real running state of the unit. The nuclear power plant primary loop equipment, such as the main pump, generates a large amount of heat when running, so that the medium in the primary loop pipeline is continuously heated. This part of heat can be transferred to the secondary loop through the steam generator and discharged through the atmospheric release valve. The method provided by the present application uses this part of heat for debugging the medium and long-term heat removal system, thereby replacing the external heat source.

[0052] The embodiment of the present application discloses a debugging system of a medium and long term heat removal system under a severe accident condition of a nuclear power plant, as shown in Figure 1

[0053] The cooling water pipeline of the cooling water side medium and long term heat removal system is connected in parallel with the cooling water side pipeline of the original equipment cooling system.

[0054] The residual heat removal system is connected with the primary loop, and comprises a residual heat removal pump 4 and a residual heat removal system heat exchanger 5.

[0055] In the cooling water side medium and long term heat removal system, the cooling water pipeline is connected with one side of a medium and long term heat removal system cooling water heat exchanger 9, and forms a first circulation loop with the residual heat removal system heat exchanger 5.

[0056] Second and third valves 14 and 15 are respectively arranged on the pipelines on the two sides of the medium and long term heat removal system cooling water heat exchanger 9 in the first circulation loop.

[0057] The original equipment cooling system comprises a heat exchanger and a connecting pipeline, and the heat exchanger is an original equipment cooling water heat exchanger 6.

[0058] The heat exchanger forms a third circulation loop with the residual heat removal system heat exchanger 5.

[0059] First and fourth valves 13 and 16 are respectively arranged on the pipelines on the two sides of the residual heat removal system heat exchanger 5 in the third circulation loop.

[0060] The cold water output by the heat exchanger is subjected to heat exchange in the residual heat removal system heat exchanger 5, the hot water obtained through the heat exchange is returned to the heat exchanger, and the cold water after the heat exchange in the heat exchanger is again subjected to heat exchange in the residual heat removal system heat exchanger 5.

[0061] The other side of the medium and long term heat removal system cooling water heat exchanger 9 is connected with a ventilation cooling tower fan 12 through a pipeline to form a second circulation loop.

[0062] A mechanical ventilation cooling water circulation pump 11 is arranged on the second circulation loop.

[0063] The primary loop is composed of a reactor 1, a steam generator 2, a main pump 3 and connecting pipelines.

[0064] ​The heat generated by the primary circuit is discharged to the residual heat removal system, the heat received by the residual heat removal system is exchanged at the residual heat removal system heat exchanger 5, and the heat after the heat exchange is transmitted through the third circulating loop; or the first valve 13 and the fourth valve 16 on the third loop are closed, the heat discharged from the residual heat removal system heat exchanger 5 is ensured to be completely transmitted to the cooling water side medium and long term heat removal system, and then is radiated to the air through the cooling tower; that is, is radiated to the air through the first circulating loop and the second circulating loop.

[0065] The steam pipeline of the steam generator secondary side passive cooling system is connected with the main steam pipeline of the steam generator, and the condensate water pipeline is connected with the feedwater pipeline of the steam generator.

[0066] The passive cooling system heat exchanger 19 is connected with the steam pipeline and the condensate water pipeline respectively.

[0067] The steam pipeline is also connected with the nitrogen pipeline, the fifth valve 17 is arranged on the steam pipeline, the sixth valve 18 is arranged on the condensate water pipeline, and the seventh valve 20 is arranged on the nitrogen pipeline.

[0068] Based on the above debugging system, the embodiment of the application further discloses a debugging method of a medium and long term heat removal system under a severe accident condition of a nuclear power plant, which comprises the following steps:

[0069] The debugging of the cooling water side medium and long term heat removal system comprises the following steps:

[0070] Step 1-1: installation and preparation are performed.

[0071] In the design stage or the later modification of the nuclear power plant, the cooling water pipeline of the medium and long term heat removal system is connected with the equipment cooling water pipeline originally designed in the power plant.

[0072] The selection of the connection point should ensure that the residual heat removal system heat exchanger 5 can be directly or indirectly cooled by the cooling water of the medium and long term heat removal system, that is, the requirement in the drawing is met. Figure 1

[0073] The first circulating loop and the second circulating loop are filled with water and exhausted.

[0074] The circulating cooling water is established for the first circulating loop and the second circulating loop, specifically, the circulating pump 10 and the mechanical ventilation cooling water circulating pump 11 are started.

[0075] The ventilation cooling tower fan 12 is started.

[0076] It is confirmed that all the devices of the cooling water side medium and long term heat removal system are started without any abnormality, and the circulating cooling water has been established.

[0077] ​Step 1-2: In the late stage of the nuclear power plant unit heat test, the one loop temperature is gradually reduced to the temperature at which the residual heat removal system operates or as close as possible to the upper limit of the spent pool cooling temperature under severe accident conditions, so as to simulate the real working condition under the unit state;

[0078] Step 1-3: When the one loop temperature is reduced to the temperature at which the residual heat removal system operates, the residual heat removal pump 4 is started;

[0079] Step 1-4: When the one loop temperature is close to the design maximum temperature of the spent pool under severe accident conditions, the first valve 13 and the fourth valve 16 are closed, and the second valve 14 and the third valve 15 are opened, so that heat exchange is realized between the residual heat removal system heat exchanger 5 and the cooling water pipeline of the long-term heat removal system;

[0080] That is, the cooling water of the residual heat removal system heat exchanger 5 is modified from the original equipment cooling water system to the long-term heat removal system cooling water cooling;

[0081] Step 1-5: The atmospheric release valve in the secondary loop is closed, and the heat generated by the main pump 3 is as much as possible transmitted to the long-term heat removal system through the residual heat removal system heat exchanger;

[0082] Step 1-6: Check the one loop temperature change, check the residual heat removal system heat exchanger 5 inlet and outlet temperature, check the long-term heat removal system temperature and equipment operation on the cooling water side;

[0083] The debugging of the steam generator secondary side passive cooling system includes the following steps:

[0084] Step 2-1: Installation and preparation:

[0085] In the design stage or later modification of the nuclear power plant, the steam pipeline of the steam generator secondary side passive cooling system is connected with the main steam pipeline, and the condensate water pipeline is connected with the feedwater pipeline;

[0086] The pipeline from the passive cooling system heat exchanger 19 to the sixth valve 18 is filled with desalted water;

[0087] The pipeline from the fifth valve 17 to the passive cooling system heat exchanger 19 is filled with nitrogen;

[0088] Step 2-2: Confirm that the one loop temperature is close to the real operating temperature during the unit heat test, and close the atmospheric release valve;

[0089] Step 2-2: Open the fifth valve 17 and open the seventh valve 20 to discharge the nitrogen filled in the pipeline;

[0090] Step 2-2: Open the sixth valve 18 after a period of time;

[0091] Step 2-2: Monitor the water level change of steam generator 2 and the primary loop temperature;

[0092] Step 2-3: Determine the heat exchange efficiency of the passive cooling system heat exchanger 19 by calculating the heat release of the main pump, the steam pipeline flow, the condensate flow, and the temperature.

[0093] The above description of the embodiments is only used to help understand the method of the present application and its core idea. It should be noted that for those skilled in the art, some improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

[0094] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A commissioning system for a medium- to long-term heat exhaust system under severe accident conditions in a nuclear power plant, characterized in that, The primary loop consists of a reactor, a steam generator, a main pump, and connecting pipelines. The waste heat removal system is connected to the primary loop and includes a waste heat removal pump and a waste heat removal system heat exchanger. The cooling water pipeline of the medium- and long-term heat exhaust system on the cooling water side is connected in parallel with the cooling water pipeline of the original equipment cooling system; In the medium-to-long-term heat exhaust system on the cooling water side, the cooling water pipeline is connected to one side of the cooling water heat exchanger of the medium-to-long-term heat exhaust system, forming a first circulation loop with the heat exchanger of the waste heat exhaust system. On the other side of the cooling water heat exchanger of the medium- and long-term heat exhaust system, a second circulation loop is formed with the ventilation cooling tower fan through pipelines. The original equipment cooling system includes a heat exchanger and connecting pipelines, forming a third circulation loop with the waste heat discharge system heat exchanger. The cold water output from the heat exchanger undergoes heat exchange in the heat exchanger of the waste heat discharge system. The hot water obtained from the heat exchange returns to the heat exchanger, undergoes heat exchange again in the heat exchanger, and the cold water that has exchanged heat re-enters the heat exchanger of the waste heat discharge system. The steam pipeline of the passive cooling system on the secondary side of the steam generator is connected to the main steam pipeline of the steam generator, and the condensate pipeline is connected to the feedwater pipeline of the steam generator. The heat exchangers of the passive cooling system are connected to the steam pipeline and the condensate pipeline, respectively.

2. The commissioning system for the medium- and long-term heat exhaust system under severe accident conditions in a nuclear power plant according to claim 1, characterized in that, On the first circulation loop, a second valve and a third valve are respectively installed on the pipelines on both sides of the cooling water heat exchanger of the medium and long-term heat exhaust system; On the third circulation loop, a first valve and a fourth valve are respectively installed on the pipelines on both sides of the heat exchanger of the waste heat discharge system.

3. The commissioning system for the medium- and long-term heat exhaust system under severe accident conditions in a nuclear power plant according to claim 1, characterized in that, The steam pipeline is also connected to a nitrogen pipeline. A fifth valve is installed on the steam pipeline, a sixth valve is installed on the condensate pipeline, and a seventh valve is installed on the nitrogen pipeline.

4. A commissioning method for a medium- to long-term heat exhaust system under severe accident conditions in a nuclear power plant using the commissioning system described in any one of claims 1 to 3, characterized in that, include: Commissioning of the medium- and long-term heat exhaust system on the cooling water side and the passive cooling system on the secondary side of the steam generator; The commissioning of the medium- and long-term heat exhaust system on the cooling water side includes the following steps: Step 1-1: In the later stage of the hot commissioning of the nuclear power plant unit, when the unit is running smoothly, the primary loop temperature is gradually reduced to the operating temperature of the waste heat removal system or as close as possible to the upper limit of the waste pool cooling temperature under severe accident conditions, in order to simulate the real operating conditions. Steps 1-2: When the temperature of the primary circuit drops to the operating temperature of the waste heat removal system, start the waste heat removal system and activate the waste heat removal pump. Steps 1-3: When the primary loop temperature approaches the design maximum temperature of the waste pool under severe accident conditions, close the third circulation loop pipeline and open the cooling water pipeline of the medium- and long-term heat exhaust system on the cooling water side. Heat exchange is achieved between the cooling water pipeline of the medium- and long-term heat exhaust system on the cooling water side and the heat exchanger of the waste heat exhaust system. Steps 1-4: Check the temperature changes in the primary loop, check the inlet and outlet temperatures of the heat exchanger in the waste heat removal system, and check the temperature and equipment operation of the medium- and long-term heat removal system on the cooling water side. The commissioning of the passive cooling system on the secondary side of the steam generator includes the following steps: Step 2-1: Use the main steam to provide heat to the passive cooling system on the secondary side of the steam generator; Step 2-2: Monitor the water level changes and primary circuit temperature of the steam generator; Steps 2-3: Determine the heat exchange efficiency of the passive cooling system heat exchanger by calculating the heat released by the main pump, the steam pipeline flow rate, the condensate flow rate, and the temperature.

5. The commissioning method for the medium- and long-term heat exhaust system under severe accident conditions in a nuclear power plant according to claim 4, characterized in that, Before step 1-1, the following is also included: Fill and vent the first and second circulation loops; Establish circulating cooling water for the first and second circulation loops; Start the ventilation cooling tower fan; It was confirmed that all equipment in the medium- and long-term heat exhaust system on the cooling water side started up without any abnormalities, and that the circulating cooling water system had been established.

6. The commissioning method for the medium- and long-term heat exhaust system under severe accident conditions in a nuclear power plant according to claim 4, characterized in that, Following steps 1-2, the following also includes: Close the atmospheric release valve to transfer as much heat as possible generated by the main pump to the medium- and long-term heat discharge system on the cold water side through the waste heat discharge system heat exchanger.

7. The commissioning method for the medium- and long-term heat exhaust system under severe accident conditions in a nuclear power plant according to claim 4, characterized in that, Before commissioning the passive cooling system on the secondary side of the steam generator. Nitrogen purging maintenance was performed on the steam lines of the heat exchangers in the passive cooling system. Fill the condensate lines of the heat exchanger in the passive cooling system with demineralized water.

8. The commissioning method for the medium- and long-term heat exhaust system under severe accident conditions in a nuclear power plant according to claim 7, characterized in that, Step 2-1 specifically includes: During the unit's hot commissioning, once the primary circuit temperature is confirmed to be close to the actual operating temperature, the atmospheric release valve is closed. The nitrogen in the steam pipeline is vented using the main steam. The main steam is used to provide heat to the passive cooling system on the secondary side of the steam generator.

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